An OVD (Optical Variable Device) is a foil formed by depositing a metal on a relief diffraction grating and has a unique optical change function such as three-dimensional image formation or color shift. The OVD is called a hologram or metal foil. There are a sheet type OVD made by die-cutting a sheet, a patch type OVD formed into a predetermined shape, and a linear thread type OVD. The OVDs are attached to a partial or entire surface of a valuable printed product such as a bank note, securities, merchandise coupon, or credit card and used as one of anti-counterfeit technologies because of their advanced manufacturing methods.
A printed product with an OVD is manufactured by attaching a sheet- or patch-type hologram to a base material by transfer or press. A thread-type OVD is contained in a base material as a watermark or attached into or onto a sheet surface by transfer or press.
In manufacturing products with OVD, defects such as a pinhole or peeling may occur upon attaching a hologram sheet to a base material due to, e.g., a hologram sheet print error, a transfer error to a base material, or maladjustment of machines. OVD attachment state inspection is an important item of quality control. It is necessary to remove a product with a defect as a defective. However, the pattern observed on the diffraction grating of an OVD changes depending on a slight change in the incident angle of light because the diffraction grating has a dependence on the wavelength and incident angle of light. It is therefore impossible to inspect a stable OVD attachment state under visible light. An inspection method or apparatus places a printed product with an OVD, acquires its image by using a camera, and executes comparison testing by displaying the image on a monitor or printing the image.
An inspection apparatus is known, which reproduces, captures, and inspects a hologram image of a holographic printed product containing a holographic region and a print region (e.g., patent reference 1).
A hologram position inspection method and apparatus are also used as an inspection method and apparatus capable of easily detecting an edge of a hologram and easily inspecting whether the hologram is applied within a tolerance (e.g., patent reference 2).
There also exist a hologram sheet inspection method and apparatus capable of automatically accurately inspecting many different kinds of holograms in a large quantity at once and continuously for each of error items including chipping of a sheet, hologram image misregistration, and unprinted areas, repetitive printing, and misregistration of hologram sheets (e.g., patent reference 3).
Patent reference 1: Japanese Patent Laid-Open No. 2000-187008
Patent reference 2: Japanese Patent No. 3095215
Patent reference 3: Japanese Patent Laid-Open No. 2002-181718